Guppies! Where?!

[random919]

Hey Guys,
My nan has a 65L tank with 3 cories, 2 pakistan loaches, 3 neon tetras, 2 black tetras, 3 mollies (juvinile ones) and supposedly 4 guppies. However, we have noticed that one by one, the guppies are disappearing. They swam to the top of the tank and then within half a day, dead. We fished out a dead one today and one yesterday. All the other fish are fine and swimming around without a problem. They are all not affected. No new fish have been added for about a month or so and that was one mollyfrom my tank as he needed a new home.
We checked the water and the results were:
pH - 7.5
Nitrate - 20
Nitrite - 0
Ammonia - 0
We have an Aquaflow 200 pump and the heater is on for about 25 degrees C. We do reasonably regular water changes of about 15 - 20% every week - 10 days as my nan needs help to do it. They are fed twice a day with flakes and the light is on for about 8 hours a day.
What do you think could be wrong and why are they dying?
Thanks
Random919

 

[phoenixgsd]

pakistan loaches can grow quite big they also can be agressive i wouldnt be surprised if they were the culprit, i would also say your nans tank is overstocked

 

[Tuxyu3]

I agree about the Loaches, they should really be kept in groups (not that i would recommend adding more to this tank) of at least four or they can become insecure and aggressive.

 

[phantomlink]

Also Pakistan loaces ( better known as YoYo loaches ) grow to be 5 inches, and are incredibly agile and need alot of space, that tank is way too small